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Lens
If you are looking for a specific lens, you can browse by lens, by lens maker or by lens mount. A lens (here: photographic lens, also known as objective lens or photographic objective) is an optical device through which light is focused in order to form an image inside of a camera either on film or on a digital sensor. Anatomy of a Lens Elements An element of a photographic lens is an optical lens itself, but one consisting of one single round piece of honed glass or plexiglass. Most photographic lenses have several different elements combined in the lens barrel, all having the same optical axis. Two or more elements which are cemented together (without an air space between them) make a group. (Older lenses used Canada balsam, made from pine pitch, for cement, while newer lenses use higher-tech UV-cured cements.) surfaces Making a lens element is not as simple as just having a piece of founded glass. That's just the beginning. After having founded a an appropriate piece of glass its surfaces must be ground until their plane or spherical concave or spherical convex surfaces are according to the lens elements' mathematically calculated geometry. If a surface has to be aspherical the grinding process becomes more complicated. After grinding the surfaces must be polished since they must be transparent. Nowadays one, some or even all element surfaces of a lens get a transparent special coating against reflections between the elements. The elements are classified as converging (light-bundling) and diverging (light-spreading) elements basic material The basic materials of lens elements is either optical glass or transparent plastic material like acrylic glass or plexiglass. Some microscopes use oildrop lenses, and some lens constructions have elements filled with water. An exception are the mirror elements of some super-tele lenses which can be made of other materials. crown glass ... Crown glass is a very old kind of glass, once blown into a "crown" to achieve the round elements of medieval glass windows. Its better variants were also used as optical glass. ... and flint glass Flint glass is made with a high allotment of lead. Its prismatic division of colors is quite different from that of crown glass. That effect made the construction of color-corrected lenses possible by combining elements of both types of glass so that each neutralizes the prismatic color dispersal effects of the other. The first renowned achromatic lens construction was found 250 years ago by the English optician John Dollond on the basis of his own research and maybe little help of the Swedish expert Samuel Klingenstierna. Barrel The barrel is the tube-shaped outer shell that contains the lens elements. Aperture The aperture is a round opening in or behind a lens (or between elements of it) that limits the amount of light passing through it into the camera. The lens's optical axis passes in 90 degree angle through the centre of that hole. The term aperture is also often used to describe the amount of light transmitted by the lens, for which the size of this opening is one determining factor. It is usually adjustable, automatically or by turning an aperture control, usually but not always on the lens barrel. A diaphragm is a round aperture of variable size. Mount A lens mount is the part of an interchangeable lens that connects to the camera body. Geometry of a Lens Optical Axis The optical axis is an imaginary straight line which passes through the centers of curvature of the lens elements and meets the (untilted) focal plane at a 90-degree angle Optical Center The optical center is the point on the optical axis where it passes through the (theoretical one) plane of refraction of the lens Focal Plane The focal plane is the plane onto which a lens projects the image of the focused image subject. Usually it's flat but especially some old bakelite cameras with just a simple meniscus lens have a curved image plane since the "curvature of field" of such lenses is stronger than that of more sophisticated multi-element lenses which deliver more or less "planar" images. Usually the middle of a focal plane sits at a 90-degree angle to the optical axis, except when tilt/shift movements cause deviations from a camera's normal geometry of light-pathes. The position of the film's or digital sensor's light sensitive surface should be identical with the focal plane. Focal Length The focal length of a lens is the distance on the optical axis from the focal plane to the optical center of the lens when it's focused to infinity. Fixed and Interchangeable Lenses Fixed Lenses A fixed lens is simply a lens that is permanently fastened to its camera as opposed to a system camera that allows different lenses to be used on the same camera easily. Fixed lenses are commonly found cameras aimed at consumers, from old box. There are certain advantages to having a fixed lens on your camera: As no mechanism for changing lenses needs to be built into the camera design it can help keep the camera smaller and lighter. A fixed lens means that there is less chance of introducing dust to the sensor surface.Fixed lenses are designed for a specific camera model and so fewer compromises have to be made in the lens design. Cost - if your camera comes with a fixed lens you don't have to worry amount buying a lot of additional glass to build a system. Portability - a fixed lens should be enough for most situations you encounter so you have less accessories to carry and you will waste less time changing lenses. Interchangeable Lenses Interchangeable lenses are more commonly found on cameras aimed at professionals and enthusiasts including large format. The advantages to interchangeable lenses include: A larger range of focal lengths and specialties (shift, macro, etc.) are available than you are likely to find on any fixed lens camera. Each lens can be designed for a specific kind(s) of working situations and specialties without the compromises a generalist fixed lens has to be designed for. Longevity - you can upgrade your camera body without losing any investment you have made in additional lenses if your new camera choice is in the same family as your old camera. Macro means that macroscopic exposures are possible since these lenses allow very near image subject distances. Wide-angle lenses may allow distances of 20 cm. On most zoom lenses near distances cannot be chosen directly. Those lenses have to be switched to a special macro mode. Modern digicam zoom lenses have macro modes for minimal image subject distances between 1 (!) and 10 cm. Lenses with a longer tube elongation added may allow shorter distances. Such elongations are usually reached with supplementary macro bellows or macro rings. On top of a macro bellows nearly any sort macro lens head, normal lens, wide-angle lens or zoom lens can be used for macro photography, even if the lens is not explicitly sold as macro lens. For macro lenses or lens/elongation combinations the maximum reproduction scale (or reproduction ratio) is a characteristic parameter. For example a reproduction scale of 1:3 means that the object focused in shortest distance will be reproduced in one third of its original size. Only on a print will it appear enlarged. A reproduction scale of 2:1 means that the object focused at the shortest focusing distance will be reproduced in twice its original size. It will appear enlarged on the negative. Repro Repro lenses are not constructed for speed but for sharpness. Many are made for making images of frame sizes greater or equal 9×12cm. And many allow short subject distances. These lenses are ideal for reproducing two-dimensional objects. Used on long bellows repro lenses are good macro lenses. Another usage is making images of still objects in studios. Naturally repro cameras need this sort of lens. Many repro lenses, even some for smaller frame formats, are constructed apochromatic. Adapter/Converter Some lenses are optical adapters allowing to use cameras with microscopes, endoscopes or telescopes. Others are converters to be mounted between camera and lens to give the lens twice of its original focal length. Lens Speed Lenses that let in a lot of light are called 'fast' lenses. This quality is indicated by a number which is called the maximum aperture or maximum f stop. The smaller the number, the faster the lens. "Fast lens" is a relative description: It depends on the image format and the focal length. Considering 35mm film 24x36mm frame format, a fast lens with fixed focal length between 24mm and 135mm will typically have a maximum aperture of f2.5 or less. f2.8 200mm lenses and f3 300mm lenses are fast lenses, while the same focal lengthes combined with maximum aperture of f2.0 are exceptionally super fast. Fast zoom lenses have a maximum aperture of at least f2.8 plus max. aperture f4.5 in tele mode. f1.8 for focal length 50mm and f2.8 for other focal lengthes between 24mm to 135mm are the standard lens speeds for 35mm format lenses. f2.8 is already fast lens speed when regarding medium format lenses. f1.4 is a standard lens speed for 16mm movie camera lenses. The max. aperture number or lens speed is equal to the largest aperture's diameter divided by the focal length of the lens. As lenses get faster, they become larger, more difficult to make, and more costly. Fast lenses are important if you want to take photos in dim light without flash, and without a tripod or if you need a shallow depth of field. Depth of field The depth of field is a way of describing how much of your image is in focus. When the camera is focused at a certain point, it will remain in focus for objects slightly in front of that point, as well as slightly behind. The distance between the closest object that is in focus, and the most distant one, is the depth of field. The depth of field is dependent purely on the geometry of the lens, and cannot be changed by the manufacturer. Generally, the shorter the focal length of the lens, the greater its depth of field. Zoom lenses have more depth of field when set to their shortest focal length, than when set to the longest. Since all small digital cameras have lenses with very short focal lengths, they tend to have very large depth of field. This has many benefits, and generally makes the job of the autofocus mechanism much easier. On the other hand, certain aesthetic effects become more difficult when the lens has too much depth of field, for example having a sharp subject emphasized over a blurry background. Finally, the faster the lens, the lower the depth of field. This means that while using a very fast lens will allow you to photograph in dim light, it will be very difficult to adjust the focus when you do this. Moral: if you think you want a very fast lens, you will pay for it in cost, weight, bulk, and poor depth of field. But of course you will be able to take pictures in dim light, and to detach your subject from the background in more situations. * Depth of field explained by Ching-Kuang Shene * In French: Depth-of-field chapter of the Photography wikibook Distortion This is when the lens represents straight lines as bent. This can be often seen in zoom lenses at both ends of the zoom range, straight lines at the edge of the frame will appear slightly curved. There seems to have arisen a kind of accepted dogma that this is a bad thing, though nobody seems to explain why. In fact it depends on the type of pictures you will take. It will usually be bad with architectural pictures, with pictures full of geometrical shapes, and for the reproduction of flat objects such as paintings. It will usually not matter for portraits and for daily shooting. If you think it matters for you, then check carefully for that behavior in any lens you are choosing. Once again, it seems to be difficult for lens manufacturers to achieve very low distortion in conjunction with all the other good features they want their lenses to have. The lenses on which distortion seems the most difficult to correct are the wide angle lenses. Barrel Distortion When straight lines bow out towards the edge of the frame (like the profile of a barrel) it is known as barrel distortion. This is typically found to some extent at the wide end of many zoom lenses. Pin cushion Distortion When straight lines bow in from the frame edge it is known as pin-cushioning. This is typically found at the long end of zoom lenses. Lens fault corrections Astigmatism Astigmatism (pointlessness) is when a point sending light through a lens cannot be projected as one point behind the lens. It appears as a line on the focal plane. Another explanation is that astigmatic lenses cannot project horizontal lines into the same image plane as vertical lines. That effect mainly appears when biconvex or biconcave lens elements are used. In the case of the biconvex lenses in our eyes astigmatism can be corrected by a lens element with reverse astigmatic effect (cylinder lens). The only possible correction of astigmatism of camera lenses is to combine at least 3 lens elements. The different elements of a well-constructed triplet minimize astigmatism. For more than one hundred years most photographic lenses are anastigmatic, but the term was still used for marketing camera lenses until the 1950s. Chromatic aberration Chromatic aberrations are reduced by using elements made of different varieties of glass. Elements made of different glass help to bundle red, green and blue light that is coming from one single point in front of the lens in one single point behind the lens. Lens designs which are projecting blue and green light focused unified are called achromatic lenses. When the focusing correction of the lens includes blue, green and red light it's an apochromatic lens design. Coating against reflections between lens groups Modern lenses are coated with a very thin layer of antireflective material (like magnesium fluoride or calcium fluoride). This lens coating is applied to the each element of a lens which has a surface exposed to air. The only purpose of coating lenses is to reduce lens flare by eliminating reflection off the surface of the glass; this has the effect of increasing contrast and giving images more "punch". Lens coating thicknesses are typically of the order of a few wavelengths of light, - a few tens of nanometers (nm). Lens coatings have nothing to do with color correction of lenses, as is widely thought. An uncoated surface will exhibit a reflection ~4% of the incident light, in a three element lens (with six surfaces) this represents a transmittance of ~78%. A Magnesium fluoride coated surface would exhibit a reflectivity of ~1% so the six surface system may have a transmittance of ~94%. Multicoating refers to the application of more than one layer of coating on a lens. Multicoated lenses may have might higher transmittances that single coated lenses, and may be specifically tailored for use with a small number of frequencies. Coatings on lenses may used to enhance scratch resistance. All modern lenses are coated. Coating gives lenses their colored reflective look. Since lens coatings are relatively fragile, one should always be careful when cleaning coated lens surfaces (like the front of the lens), so as not to scratch or smudge them. Many photographers keep a daylight or UV filter on the lens to protect its surface and avoid the necessity of frequent cleaning. * For an explanation of how lens coatings work, check here. Lens design According to this source http://www.optics.rochester.edu/~stroud/BookHTML/ChapX_pdf/X_72.pdf of Rochester University ("Wither Optical Design?", by Douglas Sinclair) lens design reached its heyday in the 20th century. It was the task of the "traditional" lens designers to "balance the aberrations of centered optical systems to achieve a maximum image quality". The optical engineer "worked on layouts and negociated with the marketing and mechanical departments to get enough room for the design to be implemented within the laws of physics." On the one hand Sinclair predicted that the optical engineering will take over the lens designers' tasks in the 21st century. On the other hand he reported from the 1998 lens design contest of the renowned International Optical Design Conference that the top-five designs were made by experienced lens designers. All of them used lens design software. Sinclair's prediction may be wrong: The contest proved that modern lens designers can work with software tools which may be developed by optical engineers, but as Sinclair himself wrote the good lens designs are more than the software can generate. According to his writings it's almost an art to add practical lens design experience to purely technical solutions. What may have changed is that both, optical engineers and lens designers need broader understanding of the whole related fields of physics, geometrical optics, wave optics, algorithms, etc. . see also Category:Lens designers See also * Lens mount * Lens comparisons * Lensbabies Links * Explanation with photos, Courtesy of toxpose.com * In Praise of the Standard Prime Lens * fisheye lens database * macro lens database * Camera Lens Review * Simple lens tests are found at this Kiev 60 page * history of small format film lenses * Interchangeable non-AF lenses: an attempt of clasification, at Manual Camera Lens aberrations * aberrations, explained on homepage of JML * lens triplet in wikipedia * shareware document of Darko Vasiljevic about triplet optimization * Category:Camera parts